Then you could use the following code to get all projects that have a matching tag in either field:
// add the filter
if(!empty(param('filter'))) {
$filter = param('filter');
$projects = $projects->filter(function($project) use($filter) {
// check if one of the fields contains the tag
if(in_array($filter, $project->filter()->split(','))) return true;
if(in_array($filter, $project->job()->split(','))) return true;
// neither field contains the tag
return false;
});
} else {
$projects = new Pages(array($projects_portfolio_featured, $projects_side_featured));
}
return compact('projects', 'filters', 'filter');